1. Field of the Invention
The present invention relates to a dielectric filter and a dielectric duplexer each having a plurality of inner conductors formed in a dielectric block, and having an outer conductor formed on the outer surface thereof, and to a communication system using them.
2. Description of the Related Art
A well known dielectric filter structure is shown in FIG. 12. This dielectric filter has a dielectric block 1 having a substantially rectangular prism shape. Three inner conductor forming holes 2a, 2b, and 2c are formed from one end face to the opposite end face of the dielectric block 1, and inner conductors 3a, 3b, and 3c are formed on the respective inner surfaces. A pair of input-output electrodes 5a and 5b are provided on the outer surface of the dielectric block 1. An outer conductor 4 is formed over substantially the entire outer surface except the areas where the input-output electrodes 5a and 5b are formed. Each of the inner conductor forming holes 2a, 2b, and 2c is a stepped hole, and each of the inner conductors 3a to 3c has a gap g in the larger inner-diameter portion, in the vicinity of the end face, and this portion constitutes an open end of a resonator formed by the corresponding inner conductor.
In this dielectric filter, a xcex/4 resonator is formed by each of the inner conductors 3a to 3c. This dielectric filter is constructed with inductive coupling between these resonators by arranging the smaller inner-diameter portions eccentrically, and by stray capacitance generated in the gaps g, whereby this dielectric filter has an attenuation pole in a higher frequency area of the pass band.
The conventional construction as described above, however, can not always achieve sufficient out-of-band attenuation. Accordingly, various methods have hitherto been adopted for improving the out-of-band attenuation characteristics, such as providing a coupling element or a coupling substrate to provide resonance poles, increasing the number of stages, adding another filter, and the like. However, these methods raise problems, in that they increase the number of components, increase the filter size, and increase the cost. In addition, the pass band characteristics can be significantly changed.
The present invention can avoid these problems and provide a dielectric filter and a dielectric duplexer capable of improving the out-of-band attenuation characteristics in the vicinity of the pass band without changing the pass band characteristics, and a communication system using the dielectric filter and duplexer.
The dielectric filter and dielectric duplexer in accordance with the present invention comprise a dielectric block having a substantially rectangular prism shape; a plurality of inner conductor forming holes arranged between the opposite end faces of the dielectric block; and a plurality of inner conductors formed on the inner surfaces of the plurality of inner conductor forming holes; and an outer conductor formed on the outer surface of the dielectric block. In this dielectric filter, on at least one side surface of the dielectric block, substantially parallel to the longitudinal axes of the inner conductors, an elongated outer conductor gap is provided in at least one portion of at least one area between a pair of adjacent inner conductors.
The outer conductor gap is arranged so as to block ground currents flowing between the two input/output electrodes. Thus it may be arranged generally transverse to the ground currents. Strict parallelism with the inner conductors is not absolutely necessary, but is considered to be preferable.
The gap may extend between the opposite end faces or only part way across the dielectric block. The gap may have various shapes. More than one gap may be provided.
In accordance with the above-described features, since one or more outer conductor gap is provided at an area between a pair of adjacent inner conductors, the degree of coupling between the pair of resonators is reduced and the influence of the TEM mode is suppressed. Thus, the out-of-band attenuation in the vicinity of the pass band can be increased substantially without changing the pass band characteristics, as described hereinafter. The reason for this is considered to be as follows. Providing outer conductor gaps on the outer surface of a dielectric block changes the ground current flowing through the outer conductor and thereby changes the attenuation characteristics. However, providing them exclusively in areas between adjacent inner conductors influences the degree of coupling between resonators very little, and thereby hardly changes the pass band characteristics. That is, required filter characteristics can be easily realized by a simple method in which an outer conductor gaps are provided at predetermined locations on the outer surface of the dielectric block.
Furthermore, since a communication system in accordance with the present invention is constructed of the dielectric filter or the dielectric duplexer having the above-described features, it is inexpensive, small-sized, and superior in its characteristics.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.